Analysis of Methane and Carbon Dioxide Fluxes in Forest Soils Using Concentration and Stable Carbon Isotope Ratio Gradients

The importance and mechanisms of methane/carbon dioxide exchange between forest soils and the atmosphere continue to be debated. We have conducted summer fieldwork in 2005 and 2006 to collect and analyse soil gas samples from forest locations in Quebec and British Columbia. Carbon isotope ratios and concentration soil profiles for both CH4 and CO2 are determined using GC-IRMS to clarify carbon fluxes within these forest soils. Initial observations of δ13C and concentration gradients of CO2 show a increase in [CO2] and decrease in δ13CO2 with soil depth. These trends are indicators of both net CO2 production in and efflux from these forest environments. In contrast, the CH4 data show increasing δ13CH4 and decreasing [CH4] with increasing soil depth. This indicates that the soils at both locations act as net sinks of atmospheric methane. Chamber flux measurements provide further support for this conclusion. They show a net accumulation of CO2 accompanied by a decrease in CH4 concentration with time in the chamber headspace. In addition, the CH4 is 13C-enriched while the CO2 is 13C-depleted in the chamber experiments. Variations present within the soil CH4 and CO2 profile indicate a non-uniform distribution of methane-oxidizing potential within the soil column. Furthermore, flux estimates using the chamber gas concentration data reveal large temporal and spatial variations in fluxes within the same location, potentially due to local heterogeneities in site characteristics.